1. Field of the Invention
The present invention relates to a liquid ejection head and a liquid ejection apparatus, and more particularly to the structure of a liquid ejection head used in an inkjet recording apparatus or the like.
2. Description of the Related Art
As an example of an image forming apparatus, there is known an inkjet recording apparatus which has an inkjet head (ejection head) having disposed multiple nozzles (ejection elements) therein, and forms an image on a medium (ejection receiving medium) by ejecting ink from the nozzles while relatively moving the inkjet head and the medium.
For an ink ejection method in an inkjet head of an inkjet recording apparatus, there is known a piezoelectric method where a diaphragm (pressure plate) constituting a part of a pressure chamber is deformed by deformation of a piezoelectric element to change the volume of the pressure chamber, ink is introduced from an ink supply path into the pressure chamber when the volume of the pressure chamber is increased, and then the ink inside the pressure chamber is ejected as a droplet from a nozzle when the volume of the pressure chamber is reduced, and also a thermal inkjet method where ink in an ink chamber (pressure chamber) is heated to generate bubbles, and then the ink is ejected with the inflation energy generated when the bubbles grow.
In the inkjet recording apparatus, the ink ejection performance affects the quality of the obtained image and the printing efficiency, and therefore the inkjet head has been subjected to various modifications to realize stable, high-speed ink ejection. In the piezoelectric method, for example, there are proposed techniques such as performing a pull ejection using the resonance between the compliance of the pressure chamber part and the inertance of the nozzle part to achieve stable, high-speed ink ejection.
Japanese Patent Application Publication No. 57-59774 discloses a driving method for an on-demand inkjet head, which ejects ink through nozzles using ejection pressure generated by piezoelectric elements, in which the volume of the pressure chamber is decreased by electrically charging the piezoelectric element in advance while the inkjet head is on stand-by prior to ink ejection. Then, during ink ejection, the piezoelectric element is made gradually discharged to increase the volume of the pressure chamber, whereupon the piezoelectric element is rapidly recharged to decrease the volume of the pressure chamber, and thus ink is ejected through the nozzle.
Japanese Patent Application Publication No. 9-327909 discloses a recording method for an inkjet recording apparatus and a head adapted to this recording method, in which a meniscus resting at a nozzle opening is rapidly drawn in such that a central region of the meniscus is displaced by a relatively large amount to a pressure-generating chamber side. When movement of the central region of the meniscus toward the pressure-generating chamber reverses, the central region of the meniscus, which is near to the pressure-generating chamber side, contracts, producing an inertial flow. The central region alone is thereby pushed out at a high speed such that an ink droplet having a smaller diameter than the diameter of the nozzle opening is ejected with stability and at a suitable speed for printing.
In the above-described related art, however, when the viscosity of the ink increases, vibration attenuation caused by the viscosity increases, and hence resonance in an even higher frequency domain (resonance point) should be used. In order to raise the resonance point, measures such as reducing the size of the pressure chamber, widening the nozzle diameter, and reducing the nozzle length should be taken.
When the size of the pressure chamber is reduced, the size of actuator provided on the pressure chamber also decreases, which may lead to deterioration in the actuator performance. When the nozzle diameter is widened, it becomes difficult to achieve the high nozzle density that is required for high-speed printing. Moreover, the nozzles that are in practical use at present are very short, and if the nozzle length were reduced even further, the rigidity of the ejection surface would decrease, causing a possible deterioration in the ejection stability. Further, when the ink viscosity increases, ink cannot be ejected unless pressure above a certain level is applied, and hence it becomes impossible to eject smaller ink droplets than the ink droplet amount corresponding to this pressure. As a result, the size of the ink droplets ejected cannot be made small.
In the driving method for an on-command inkjet head described in Japanese Patent Application Publication No. 57-59774, a technique is disclosed whereby an electric pulse is modified to enable active use of the damped vibration (resonance) of a vibration system constituted by the pressure chamber wall, an electromechanical transducing element, and the ink. However, when the ink viscosity increases, it may be impossible to maintain a predetermined ejection frequency.
In the recording method for an inkjet recording apparatus and the head adapted to this recording method, described in Japanese Patent Application Publication No. 9-327909, ink droplets having a smaller size than the nozzle diameter are ejected using the resonance phenomenon between the pressure-generating chamber and the ink to concentrate pressure in the central portion of the meniscus; however, there is no disclosure of an increase in the ejection frequency.